It is recognized that multilayer composite or "sandwich" structures offer weight and strength advantages that make them attractive materials of construction especially for aircraft panels. Such structures of the art and, likewise, the composites of this invention have a general construction as depicted in FIG. 1. FIG. 1 is not to scale as usually these composites are a centimeter or less in thickness and much longer and wider than they are thick, i.e. they are substantially two dimensional. In FIG. 1 a composite 10 is shown having a lightweight honeycomb core 11 sandwiched between two faceplates or facesheets 12 and 13. The facesheets close the open ends of the honeycomb and impart strength and rigidity to the composite. Usually two facesheets are present but, if desired, only one can be present. Facesheet 12 is shown as a two layer laminate of structural member 14 and decorative overlayer 15. Facesheet 13 is shown having the same configuration. If desired, the decorative layer can be left off of one or both of the facesheets or, if desired, an additional overcoating layer could be present with either or both of the facesheets. Generally, such composite structures have employed glass-epoxy resins as facesheets combined with a polyamide core. The various layers are held to one another with adhesives. These adhesive layers are not depicted in FIG. 1.
In the past, composite panels and aircraft lining structures have often been prepared by first silk screening a decorative surface onto a 0.005 cm polyvinyl fluoride film (Tedlar) by a continuous web process. After drying, a 0.0025 cm transparent polyvinyl fluoride film coated on one side with polymethyl methacrylate is bonded to the decorative film to provide protection for the printed surface.
This laminate is then bonded to one ply of epoxy-preimpregnated 181 E glass, and may have a surface texture impressed during the bonding operation. This provides a facesheet that is in turn bonded to the core.
One core material presently employed for sandwich paneling is an aromatic polyamide (sold under the tradename Nomex HRH-10) hexagonal-cell honeycomb structure. The cell size may be 0.312 cm, 0.625 cm, or 0.937 cm, depending upon the properties desired in the finished panel.
Structures and panels prepared from epoxy-containing materials have exhibited relatively higher heat release rates, higher mass injection rates or mass losses because of the inherent flammability of the epoxy resin facesheet material. The prior art structures evolve large quantitiies of smoke and toxic fumes when exposed to fire or heat also because of the epoxy present in the facesheets.
The present acute awareness of aircraft interior flammability problems throughout the military and civilian air-travel industry and the agencies which regulate it has led to a real need for a composite or sandwich material of construction which offers desired strength and low weight but in addition is more fire resistant than materials of the art.
Representative patents relating generally to these materials include:
U.S. Pat. No. 4,135,019 of Kourtides and Parker which relates to a composite laminate structure having a layer of glass cloth preimpregnated with polybismaleimide resin and adhered to a polybismaleimide glass or aromatic polyamide paper honeycomb cell structure which is filled or partially filled with a syntactic foam consisting of a mixture of bismaleimide resin and carbon microballoons;
U.S. Pat. No. 3,914,494 of Park which involves a high directional carbon fiber tape of an open weave construction which is impregnated with an aromati,c polyimide to give a material that may be used in a facing sheet;
U.S. Pat. No. 3,899,626 of Steffen which relates to the manufacture of a composite article having at least one layer comprising a prepolymer of bisimide and polyamine and at least one layer of a stock material; and
U.S. Pat. No. 3,811,997 of Yuan that relates to laminate articles comprised of a film of an aromatic polyimide laminated over an impregnated fibrous reinforced cloth which may optionally be laminated over a honeycomb structure.
Additional patents which are believed to show the current state of the laminate art include:
U.S. Pat. No. 3,582,447 of Slotki; PA0 U.S. Pat. No. 3,600,249 of Jackson et al; PA0 U.S. Pat. No. 3,932,689 of Watanabe et al; PA0 U.S. Pat. No. 4,052,523 of Rhodes et al; PA0 U.S. Pat. No. 4,086,378 of Kam et al; and PA0 U.S. Pat. No. 4,395,514 of Edelman.
Also of interest is commonly assigned U.S. Ser. No. 553,339, filed on Nov. 18, 1983, now U.S. Pat. No. 4,526,925 which discloses certain styryl pyridines and copolymers thereof which are used in the laminates of this invention.